To be honest, by that logic Sinbad still has more claim to being Rider than actual /Stay Night Rider does. He doesn't tame the Roc or the Elephant King, but does successfully ride them in two of his stories. She just has the Pegasus get created from her blood as she dies without any contact with it.

PKMNB0Y said their only concern was with Leviathan anyways, and I'd be happy to address whatever concerns those might be. To give a better idea of it, the full blown thing is likely not something he could summon willy-nilly, it'd only be of use in the water and he'd most likely just use its secondary effects most of the time to give him an edge/weaker attack or an aesthetic effect/show off. Like the Roc, he's also a bit too moral to summon it in situations where it might cause collateral damage anyways.


I have another Servant idea that might actually be more fun given the setting...